1. Field of the Invention
The present invention relates to a circuit board on which an analog circuit and a digital circuit are mounted. The present invention also relates to an information storing apparatus, which is equipped with such a circuit board.
2. Description of the Related Art
In recent years, the miniaturization of electrical appliances causes printed circuit boards used therein to be further miniaturize. In fact, a circuit board, in which a plurality of circuits having different functions is disposed, is also developed. Such a circuit board includes both analog circuits dealing with analog signals as well as digital circuits dealing with digital signals are, and is widely used in a printed circuit board assembly (hereinafter referred to as “PCBA”) or the like for driving an FDD.
The miniaturization of the circuit board including the analog circuit and the digital circuit is requested more and more strongly. In this case, it is occasionally necessary to dispose external conductors for analog signals and external conductors for digital signals in very close mutual position either on the surface of a circuit board or on the surfaces of different circuit boards.
Referring to FIG. 8, an example of a PCBA, in which an analog circuit and a digital circuit are disposed, will be described.
On one side of a circuit board 100, a USB FDD controller 101 including an LSI and an external memory 102 are mounted, and the USB FDD controller 101 and the external memory 102 are connected with each other via external conductors 103. Digital signals for controlling the external memory 102 are supplied from the USB FDD controller 101 and return signals are supplied to the USB FDD controller 101 via the external conductors 103. On the rear side of the circuit board 100, drive control section 110 for controlling to drive a head for the FDD (not shown) is disposed, and external conductors 111 connected to the drive control section 110 are disposed on the rear side of the circuit board 100 and extend to the side of the USB FDD controller 101, crossing the external conductors 103 on the front side of the circuit board 100.
Moreover, the external conductors 111 are connected to the external conductors 104 disposed on the surface of the circuit board 100 via through holes 112, whereas the external conductors 104 are connected to the USB FDD controller 101. Analog signals are output as drive control signals from the USB FDD controller 101 and are transmitted from the external conductors 104 to the drive control section 110 via the through holes 112 and the external conductors 111. In the circuit board 100, the digital signal conductors 103 and the analog signal conductors 111 are extending on the surfaces different from each other and cross each other.
In a circuit board 200 shown in FIG. 9, there is the same arrangement as in the circuit board 100 shown in FIG. 8, except for the external memory 102, as disposed on the circuit board 100. Similarly, the digital signal external conductors 103 and the analog signal external conductors 111 are disposed on the different surfaces.
In the stage of developing a circuit board, a firmware is mounted in the USB FDD controller 101 and/or an external memory is connected to the external conductors 103 on the circuit board 200 in the case of executing the debug, if necessary. After these works, the external memory is removed and the product is completed as a circuit board including no external memory. In case of need, the debug and/or inspection is again carried out in the state in which the external conductors 103 are connected to the external memory. In view of these facts, the external conductors 103 remain mounted on the circuit board. Even when the external memory is connected to the external conductors, digital signals are supplied from the USB FDD controller 101 to the external conductors 103.
Analog signals on the circuit board are continuously changing and the transmission of information is carried out with the aid of such change in the signals. Accordingly, the analog signals have to be transmitted with high fidelity to desired devices via the external conductors. When, however, digital signal conductors are placed on the circuit board, high frequency digital signals are transmitted thereon. Moreover, when the digital signal conductors are disposed such that they are spatially close to the analog signal conductors, the high frequency signal components resulting from the digital signal interfere with analog signals flowing in the analog signal conductors to generate noise in the analog signals. This causes to disturb a proper transmission of information. Especially in the above-mentioned circuit board for controlling the FDD, the digital signals occasionally interfere with the analog signal in an input having high impedance, such as the input from a head, thereby causing the signal to noise ratio to be deteriorated. Moreover, in the case of the circuit board including no external memory, the digital signals emanate from the digital signal conductors, so that the characteristic of reading the analog signals from the head is deteriorated.
To overcome these problems, the pattern of conductors is designed such that the analog signal conductors are not close to the digital signal conductors, or a multi-layer PCB (more than three layers), in which either different type signal conductors are disposed on corresponding different layers or the analog signal conductors detour the digital signal conductors, is employed. These procedures prevent the signal to noise ratio from deteriorating.
However, in a single layer PCB or double layer PCB, the restrictions on the circuit arrangement (the positions of the parts to be mounted, the pin arrangement of the devices to be mounted, the width of conductors and others) makes it difficult to separate the digital signal conductors from the analog signal conductors having high impedance, and therefore it is unavoidable to trade off the digital and analog properties. Furthermore, a multi-layer PCB provides an increase in the manufacturing cost of the circuit board, and therefore the restriction on the cost makes it difficult to employ such a multi-layer PCB.